Dr. D. Sattel presentation on SEG Natural Source EM for Mining Applications workshop 10/19/2018

1. A brief analysis of MobileMT data
Daniel Sattel
SEG Natural Source EM for Mining Applications workshop
10/19/2018

2. MobileMT – acquisition
- natural-field EM system designed by Petr Kuzmin and operated by
Expert Geophysics Limited (EGL)
- wide frequency range 25 Hz – 20 kHz, sampling rate 98 kHz
- records three-component airborne dB/dt data (1.4 m diameter coils)
- base station records horizontal E-field data with 2 pairs of orthogonal
sensors (signal & reference), separated by ~30 m
- cesium magnetometer (Geometrics G-822A)

3. MobileMT – acquisition

4. MobileMT – acquisition

5. MobileMT - processing
Y = admittance
not (yet) provided
Hz data currently not used, but roving tipper could be
derived from Hx, Hy & Hz data
Petr Kuzmin’s comment: Hz data is used in the
processing to derive Total Field from HxHyHz. The
expression above is a simplified expression.
Hx YxxYxy Ex
Hy YyxYyy Ey
    
=    
    
2app
DETY = ( )2
arg DETY =
DETY YxxYyy YyxYxy= −

6. MobileMT – processing, 2D case:
Parameters independent of strike direction – see also:
Pedersen, L.B. and Engels, M., 2005, Routine 2D inversion
of MT data using the determinant of the impedance tensor,
Geophysics 70, G33-G41.
( )
1
2
TE TM  = +
DET TM TEY YyxYxy Y Y= − = −
1app
app app
TE TM

 
=

7. MobileMT - Products
- Apparent conductivity grids
- 2D inversions (Occam2D, Wannamaker & Constable)
- 3D forward modeling (UBC-GIF MT3Dfwd)
- 3D inversions

8. 2D synthetic data modeling (Wannamaker & Constable)

9. 3D synthetic data modeling (UBC-GIF) – 10 km strike

10. 2D synthetic MobileMT data inversion of appcon

11. 2D synthetic MobileMT data inversion of appcon & phase

12. 2D synthetic MobileMT data modeling
Non-zero response over layered-earth!

13. 2D synthetic MobileMT data inversion

14. 2D synthetic MobileMT data inversion

15. 2D synthetic MobileMT data modeling - appcon
Target without/with IP - tau=0.001s m=1.0 fc=0.5

16. 2D synthetic MobileMT data modeling – phase
Target without/with IP - tau=0.001s m=1.0 fc=0.5

17. 2D synthetic MobileMT data inversion, ignoring IP

18. MobileMT survey – VMS exploration, N Ontario
Apparent conductivities

19. VTEM
MobileMT Con at 150 m depth
(Kaminski et al., 2016)

20. VTEM data modeling (Kaminski et al., 2016):

21. Occam2D inversion results

22. Occam2D inversion results

23. Occam2D inversion results

24. Occam2D inversion results

25. Occam2D inversion results

26. Holdsworth Gold project, N Ontario
Structural mapping - shear-zones, quartz veins

27. Holdsworth project

28. Holdsworth project

29. MobileMT appcon vs ZTEM tipper data
Pros:
- Extending frequency nice, though not critical
- dBx/dt & dBy/dt stronger signal than dBz/dt, requiring
smaller Rx coils (1.4 m) than ZTEM (7.4 m)
- combination of H/E-fields makes data sensitive to
resistivity values (eg LE), rather than resistivity contrasts
Cons:
- being more sensitive to local resistivities makes modeling
harder (start model!)
- harder to collect good E-field data in rocky/sandy/frozen
terrain?

30. Acknowledgements
Expert Geophysics Limited
Condor Consulting